1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the same.
2. Description of Related Art
In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to the photoconductor on which the electrostatic latent image is formed, whereby a toner image is formed (development). Further, the toner image is directly or indirectly transferred to a sheet, followed by heating and pressurization, whereby an image is formed on the sheet.
In the above-mentioned image forming apparatus, materials such as remaining toner or the like adhered on the surface of a photoconductor are removed (for cleaning) by bringing a cleaning blade or the like into sliding contact with the surface of the photoconductor on which a toner image is formed. In the case where a photoconductor is cleaned by a blade or the like, in order to reduce the adhering force of toner to the photoconductor and enhance the cleaning performance, a technique is used in which toner mixed with lubricant is applied to attach the lubricant on the surface of the photoconductor.
It is known that the amount of lubricant adhered on the surface of a photoconductor has a large influence on cleaning performance. For example, when the amount of the lubricant decreases, remaining toner or the like more easily adheres to the surface of the photoconductor, and this results in a problem that passing of toner is caused due to the adhered remaining toner that hinders cleaning, and a problem that the edge of the cleaning blade is drawn in the movement direction (rotational direction) of the photoconductor and is thus turned over.
Here, a case where the amount of lubricant decreases is described in detail. Lubricant added in toner has a polarity opposite to the toner (for example, positive polarity) so that it easily adheres to the toner, and thus, during development, a large amount of the lubricant moves to portions (negatively charged white parts) on the surface of the photoconductor where light exposure has not been performed. Therefore, when an image formation process of a low coverage (low coverage rate) is continuously (successively) performed, a large amount of lubricant, relative to the amount of the toner, moves from the developing device to the photoconductor in the state where the amount of the toner supplied to the developing device is small, and thus the lubricant in the developing device may possibly be exhausted. When the lubricant in the developing device is exhausted, there is no lubricant to be attached to the photoconductor, and the amount of the lubricant on the photoconductor is gradually reduced, thus causing the above-mentioned problems.
Under such circumstances, conventionally, a toner image (patch image) of a pattern for forcible toner ejection is formed in a non-image forming region of a photoconductor such that toner mixed with lubricant is additionally supplied to a developing device in the case where a low-coverage image formation process is performed. By forming a toner image of a pattern for forcible toner ejection, the lubricant in the developing device can be prevented from being exhausted, and in turn, the amount of the lubricant on the photoconductor can be prevented from being reduced. It is to be noted that the non-image forming region of the photoconductor is a region between image formation regions which are each used for forming a toner image to be transferred to a sheet, and the non-image forming region is generally called “interval between sheets.”
Japanese Patent Application Laid-Open No. 2000-206744 discloses an image forming apparatus that prevents image quality problems such as uneven image density and fogging. In the technique disclosed in Japanese Patent Application Laid-Open No. 2000-206744, before an electrostatic latent image is formed, the image density of an image to be formed to an image bearing member is detected, and when a value thus detected is lower than a desired value, a predetermined electrostatic latent image is formed to forcibly consume the developer in a developing device.
Japanese Patent Application Laid-Open No. 2003-76079 discloses an image forming apparatus that eliminates defects in association with degradation of developer which is caused when a developing device is driven while the toner consumption amount is small. In the technique disclosed in Japanese Patent Application Laid-Open No. 2003-76079, an image area ratio of an image to be imaged and the driving time for development of a developing device are detected, and developer is forcibly consumed in accordance with the image area ratio per unit driving time for development.
Japanese Patent Application Laid-Open No. 2005-43388 discloses a technique for preventing defects such as degradation of image quality due to fatigue and degradation of developer. In the technique disclosed in Japanese Patent Application Laid-Open No. 2005-43388, an image area ratio per unit migration length of a developer bearing member is detected, and forcible consumption of toner is controlled based on results of the detection.
Naturally, even when an image formation process of a low coverage is performed, lubricant in a developing device may not be exhausted. In this case, when a toner image of a pattern for forcible toner ejection is formed in a non-image forming region of a photoconductor, toner is forcibly consumed even when forcible consumption is unnecessary, and thus the toner is wastefully consumed, which is uneconomical and disadvantageous.
The techniques disclosed in Japanese Patent Application Laid-Open Nos. 2000-206744, 2003-76079, and 2005-43388 are merely techniques for preventing defects such as degradation of image quality in association with degradation of a developer, and do not include configurations for solving the above-mentioned problems.